1. Field of the Invention
The present invention relates to ink-jet heads for jetting ink.
2. Description of the Related Art
An ordinary ink-jet head has a plurality of nozzles aligned along one predetermined direction, and can form a linear dot row on a recording medium by jetting liquid drops of ink from the plurality of nozzles respectively.
Japanese Patent Application Laid-Open No. 2010-42555 (FIGS. 3 and 4) discloses a so-called line-type ink-jet head which has numerous nozzles aligned in a main scanning direction (a width direction of the recording medium being transported). In the ink-jet head of Japanese Patent Application Laid-Open No. 2010-42555, the plurality of nozzles to form one dot row on the recording medium are not arranged linearly along the main scanning direction, but one part of the nozzles has positional deviation from another adjacent part of the nozzles with respect to a secondary scanning direction (a transport direction of the recording medium) perpendicular to the main scanning direction. That is, it is configured that the plurality of nozzles in rows along the main scanning direction are divided into two types of nozzle groups (to be referred to as divisional nozzle groups hereinbelow) differing in position along the secondary scanning direction, and these two types of nozzle groups are arranged alternately along the main scanning direction. Further, in the inkjet head of Japanese Patent Application Laid-Open No. 2010-42555, ink supply ports (openings of a flow passage unit) are provided for the plurality of divisional nozzle groups, respectively. That is, ink is supplied independently to each of the plurality of divisional nozzle groups.
As shown in FIG. 11A, in the configuration of the ink-jet head, disclosed by Japanese Patent Application Laid-Open No. 2010-42555, in which the numerous nozzles arranged along the main scanning direction are divided into the plurality of divisional nozzle groups, if these plurality of divisional nozzle groups are connected to one common ink chamber and a common ink supply system is established, it is possible to simplify the flow passage structure. However, when such a configuration is adopted, problems as follows are to occur.
In an ink-jet head 200 shown in FIG. 11A, a plurality of nozzles 201 are divided into three divisional nozzle groups 202 (202a, 202b and 202c). Further, the three divisional nozzle groups 202 communicate with one common ink chamber 204 which is connected to an ink supply port 203. Among the above three divisional nozzle groups 202, the central divisional nozzle group 202b has positional deviation from the two divisional nozzle groups 202a and 202c to the upstream side with respect to the secondary scanning direction.
When carrying out recording on an incoming sheet of recording paper 100 transported in the secondary scanning direction, ink is jetted first from the nozzles 201 positioned on the upstream side in the secondary scanning direction. That is, ink is jetted in an order from the divisional nozzle group 202b to the divisional nozzle groups 202a and 202c. Here, the divisional nozzle group 202b which jets ink earlier carries out ink jetting with no ink flow occurring in the common ink chamber 204. On the other hand, the divisional nozzle groups 202a and 202c which jet ink later carry out ink jetting with an ink flow occurring in the common ink chamber 204 due to the earlier ink jetting by the divisional nozzle group 202b. Therefore, in the divisional nozzle groups 202a and 202c which jet ink later, since the ink flow has already occurred in the common ink chamber 204 just before their own ink jetting, pressure loss occurs at the time of supplying the ink to the nozzles 201, thereby delaying the ink supply to the nozzles 201. That is, between the divisional nozzle group 202b which jets ink earlier and the divisional nozzle groups 202a and 202c which jets ink later, since a difference occurs in the discharge or ejection quantity of liquid droplets, when one dot row is formed by the three divisional nozzle groups 202, dark color dots D1 and light color dots D2 are formed unevenly in this one dot row as shown in FIG. 11B to present a distinct shading.